Certain naturally occurring materials contain a carbonaceous component which, upon heating, will release a hydrocarbon product which is useful as a feedstock in petroleum processing. These "carbon-containing solids," such as oil shale and tar sands, including diatomite, may be pyrolyzed in reactor vessels having various designs. Following the pyrolysis of the carbon-containing solid to extract the volatile components, a "pyrolyzed solid" remains which contains a carbonaceous residue which may be burned to yield heat energy. The heat recovered from this carbonaceous residue may be used to supply heat for the pyrolysis of fresh carbon-containing solids during the process.
The inorganic residue that remains after the combustion of the carbonaceous residue is called "ash," or in the case of oil shale, "burned shale." This material is recycled in some retorting processes as "heat carrier material," i.e., the hot burned shale from the combustion is mixed with fresh oil shale and the heat provided is used for pyrolyzing the fresh shale. U.S. Pat. Nos. 4,199,432 and 4,183,800 describe processes in which the oil shale or other carbon-containing solid is pyrolyzed in a downward moving bed containing a mixture of recycled hot burned shale (used as heat carrier material) and particulate fresh oil shale. A countercurrent flow of gas passes upward through the bed removing the product vapors and entraining the finer particulate fractions of the oil shale. The fine particles and product vapors are drawn off the top of the reactor vessel, and the fine shale particles are removed from the gas stream by a separation device, such as a cyclone. Processes such as this present some problems in designing an efficient combustor for burning the pyrolyzed oil shale which is recycled as the heat carrier material.
During combustion of the carbonaceous residue in the pyrolyzed oil shale to produce heat, the physical integrity of the shale particles is changed and a substantial amount of fine grained burned shale is produced which is not suitable for use as recycled heat carrier particles. Therefore, it is usually necessary to separate this fine material prior to recycling the coarser grained particles.
In process schemes using a liftpipe combustor to burn the carbonaceous residue in the pyrolyzed oil shale, sufficient residence time is required to complete combustion and to assure a thermal equilibrium between the hot burning particles and the cooler recycle particles. Typically, a minimum residence time of 2 to 3 seconds in the combustion zone is required. If the fine shale and coarse shale particles are combusted in a liftpipe, the pipe must be of sufficient length to provide adequate residence time for all particles.
The present invention is advantageous for efficiently burning particulate pyrolyzed oil shale or other particulate carbon-containing solids where the fine grained material and the coarser grained material are separated prior to combustion and the burned shale serves as heat transfer material in the process.